Metal and metal oxide particles with nanometer scale dimensions have a wide range of uses, including (but not limited to) catalysts, pigments, polishes, ultraviolet absorbers and in ceramics. It is well known that such particles can be formed by chemical reaction of aqueous solutions of metal salts with heated, pressurised or supercritical water. In principle, this methodology offers distinct advantages over other methods of nanoparticle creation in terms of cost and viability as it allows the reaction to be performed as a continuous process. However it is difficult to perform this reaction on a commercial scale utilising current methods because existing reactor configurations do not allow the precipitation reaction to be controlled effectively leading to frequent blockage of the reactor and inadequate control of particle size and shape. Hence within this process, the design of the reactor where the water and the salt solution mix is of crucial importance to the size and properties of the nanoparticles produced.
The invention details a more efficient and versatile method of producing a range of nanoparticles of metal and metal oxides that could be catalytically active, and thus clearly possesses industrial applicability.
Particle size can be important for catalytic processes and other uses, and is dependant on the nature of the metal and also the intended application. For example commercially useful cerium oxide (from Johnson Matthey) has a surface area of 250 m2/g whereas silver particulate with a lower surface area, 60-100 m2/g, is also commercially useful. Without optimisation, the reactor of the invention has produced particulates of CeO2 with surface areas of 100 m2/g. This could, in principle, be improved considerably with additional work focussed on lowering the particle sizes produced by adjusting the operating conditions and metal salt concentrations.
Whilst the surface area of a catalyst is very important, the physical nature of the particles can also determine their success in the intended application. For example, zirconium oxide nanoparticulates are often amorphous in structure, which is not an appropriate form for many catalytic applications. The reactor of the invention has prepared crystalline ZrO2, which is much more useful.
Supercritical fluids, and particularly supercritical water, have been used to produce metal nanoparticles (Adschiri, Kanazawa et al. 1992; Adschiri, Hakuta et al. 2000; Galkin, Kostyuk et al. 2000; Adschiri, Hakuta et al. 2001; Cabanas, Darr et al. 2001; Cote, Teja et al. 2002; Hao and Teja 2003; Viswanathan and Gupta 2003; Viswanathan, Lilly et al. 2003) however the existing methodologies all use variants on either a T- or a Y-shaped reactor (FIG. 1).
A major limitation of these methods is that the location of the precipitation of the particles is not controlled. Particles are known to precipitate readily in reactor pipework, especially inlet pipes. The T piece reactors have been found to block frequently at the denser fluid inlet, resulting in costly and inconvenient down time being required for reactor cleaning and reassembly. These blockages can occur within minutes of the denser fluid feed reaching the T piece. Additionally, if the system is under pressure there are obvious health and safety implications associated with frequent blockages (i.e. increased risk of explosion). The invention consists of a novel design of reactor that largely eliminates these problems.